Multiple protocol transponder

ABSTRACT

A multi protocol transponder for a communications network having a antenna for receiving a first RF signal transmitted according to a communications protocol and a detector for identifying the communications protocol from the first RF signal. A protocol controller is provided for executing the identified communications protocol to generate a second RF signal. Thereafter an antenna transmits the second RF signal according to the communications protocol either by active transmission or modulated backscatter. The transponder may implement a variety of wide area and localized lane based protocols for Automatic Vehicle Identification systems such as CVO and Toll systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of commonly owned U.S. patent application Ser. No. 09/829,237, filed Apr. 9, 2001, which claims the benefit of Canadian patent application no. 2,307,361 filed on May 1, 2000.

FIELD OF THE INVENTION

The present invention relates to short range communications networks between a reader and a transponder and more particularly to a transponder for operation in a variety of such networks implementing different communications protocols.

BACKGROUND OF THE INVENTION

Communications systems for the remote identification of objects by electronic means are known in the art. The purpose of such systems may be Automatic Vehicle Identification (AVI) for Commercial Vehicle Operations (CVO) and for Electronic Toll and Traffic Management (ETTM) applications. The objectives of CVO services are to increase productivity of commercial vehicle regulatory agencies and commercial vehicle operators, and to enhance the safety of CVO drivers and vehicles. Examples of CVO services include automated permit and registration acquisition, vehicle performance monitoring, and hazardous materials incident response. ETTM allows drivers to pay highway tolls without stopping, and allows traffic managers to use transponders as probes in high traffic volume areas to facilitate incident detection. These systems provide a two-way communications means between a reader and a transponder (or “tag”). The tag can store information of interest such as identity, fuel level, time of day, cargo ownership and vehicle type, etc. This information may change and be updated as conditions change. Subsequent reading of the tags can keep those persons monitoring the vehicle, cargo or container etc. appraised of the conditions.

Such communication systems use RF signals to communicate between a reader device, such as fixed Roadside Equipment (RSE), and a mobile transponder generally fixed to the object of interest. Some systems permit both “read” and “write” capabilities, permitting a reader to access stored data in the transponder and permitting the transponder to update the data stored therein in response to signals from the reader (eg. to write the time and place of entry onto a toll highway in the transponder useful for calculating tolls based on distance and time-of-day travel).

In operation, the reader sends a RF signal to the transponder. An antenna at the transponder receives the signal from the reader and responds thereto according to one of many developed protocols. The transponder produces a series of signals in conjunction to its identity code, providing the reader with data stored in the transponder which the reader decodes.

Reader-transponder technologies divide into two basic physical modes of operation: active transmission (Active) or modulated backscatter (Backscatter). Active systems utilize a transponder with an active transmitter which responds to interrogations from the reader with an active modulated RF response signal generated by the transponder. In contrast, Backscatter systems utilize a transponder that responds to a continuous wave (CW) RF signal generated by the reader. The tag responds by modulating the continuous wave, electrically switching the tag's antenna from a reflective to an absorptive characteristic according to the tag's modulating signal. While Backscatter systems are typically limited to using amplitude modulation for the response, Active systems may use phase, frequency or amplitude modulation.

As a result of the frequency limitations of Active systems and the fact that such systems cannot employ a multiple number of frequencies within an assigned band, interference between closely located systems is typically controlled by Time Division Multiple Access (TDMA) of the closely located systems. This contrasts to the ability of Backscatter systems to use a number of possible frequencies within an assigned band with less risk of interference between adjacent capture zones. Backscatter systems, therefore, are generally not closely synchronized in time. Both systems however can employ time and frequency multiplexing to control interference.

A natural consequence of these physical differences is that the communications protocol that is most commonly used for either system is also characterized as either Asynchronous or Synchronous. Backscatter systems are typically isolated primarily in frequency instead of time, hence tolerating communication lengths that are uncontrolled and operating asynchronously. Active systems are primarily isolated in time instead of frequency, hence operating in synchronous mode with tightly controlled packet lengths.

Among the developed communications protocols are:

-   -   1) various public TDMA protocols for Wide Area or Lane Based         operations (See too, PS111-98 Standard Provisional Specification         for Dedicated Short Range Communication (DSRC) Physical Layer         Using Microwave in the 902 to 938 MHz Band, AMERICAN SOCIETY FOR         TESTING AND MATERIALS, ASTM Subcommittee E17.51 on Dedicated         Short Range Communication, West Conshohocken, Pa.);     -   2) State of California Code of Regulation(CALTRAN) Title 21(T21)         protocol (eg.         http://www.dot.ca.gov/hq/traffops/elecsys/title21/docs/t21updat.htm);         and     -   3) proprietary IAG (northeastern Interagency Group (IAG) members         (NY, NJ, PA, DE)) protocols. See for example, U.S. Pat. No.         4,870,419 of Baldwin et al. issued 26 Sep. 1989 entitled,         “Electronic Identification System”; U.S. Pat. No. 5,132,687 of         Baldwin et al. issued 21 Jul. 1992 entitled, “Electronic         Identification System”; U.S. Pat. No. 5,164,732 of Brockelsby et         al. issued 17 Nov. 1992 entitled, “Moving Vehicle Identification         System with High Gain Antenna”; and U.S. Pat. No. 5,196,846 of         Brockelsby et al. issued 23 Mar. 1993 entitled, “Moving Vehicle         Identification System”.

U.S. Pat. No. 5,425,032, of Shloss et. al, 13 Jun. 1995, entitled “TDMA Network and Protocol For Reader-Transponder Communications and Method” also discloses a TDMA protocol.

For TDMA protocols, communications are initiated by a first RF signal transmitted by a reader to a transponder at a defined frequency. In a quiescent mode, the transponder monitors the frequency for incident RF energy of about −30 dBm. The transponder receives and decodes the signal to determine if the signal encodes Manchester Data. Thereafter, the transponder looks for a Frame Control Message (FCM) within the signal received according to the protocol. Once a FCM is determined, the tag moves to an active mode and engages in TDMA protocol communications.

For the T21 protocol, in a quiescent mode, the transponder monitors the defined frequency (eg. 915±13 MHz) for a RF wake up signal according to the protocol (eg. 33 microseconds of unmodulated RF) at a minimum RF level. RF presence of at least about −22 dBm may indicate T21 signals are present. Thereafter, the transponder awakes and actively searches for a T21 Interrogation or polling message and responds appropriately as a passive Backscatter transponder.

For IAG protocols, the transponder sniffs the defined RF band for an IAG trigger signal and wakes up to engage as an active IAG transponder. IAG trigger signals are similar to T21 triggers but differ in length and are not followed by a T21 polling message. In IAG communications, the transponder responds promptly upon receipt of a wake signal, without waiting for a polling message. The RF level required to initiate IAG operation is higher than the RF level to engage T21 activity, which is in turn higher than the RF level to initiate TDMA activity.

Comprehensive standards governing the communications between the transponder and reader, and the message sets on the transponder, do not exist. Therefore, interoperability does not exist between the equipment of different manufacturers. Interoperability, in this case, is the ability of a roadside reading or interrogation device of one manufacturer to meaningfully process the data from any given transponder mounted in a vehicle. The communications industry has been unable to agree upon a path for standardization.

Vehicles, for example, which traverse large geographic areas may be required to respond to a multitude of AVI implementations for electronic toll collection or other commercial vehicle operations purposes and can only do so by selecting the transponder appropriate to the jurisdiction and mounting the transponder to the vehicle. As the vehicle moves from one jurisdiction to another, the operator is required to select from a supply of transponders, mount the appropriate one and shield any others from being inadvertently activated and possibly interfering with the transmissions.

A dual protocol transponder implementing both IAG and TDMA Wide Area protocols (sold under the trademark Fusion of Mark IV Industries) is known in the art. This transponder provides only two synchronous and actively transmitted protocols. The transponder combines the operation of two separate transponders implementing the TDMA and IAG protocols respectively by employing separate protocol detectors and a single communications controller operable according to the detected protocol. For TDMA operation, when in a quiescent mode not engaged in communication with a reader according to a selected protocol, the transponder periodically wakes up and looks for (i.e. samples) RF above a TDMA baseline signal strength and within a designated frequency range. If such an RF signal is detected, the transponder examines the signal to see if it is a TDMA protocol signal. If the detector finds an expected Frame Control Message encoded in the signal, the transponder enters TDMA communications with the reader and otherwise continues monitoring for appropriate signals. Employing a separate detector in parallel for IAG protocol detection, the transponder continuously monitors for RF signals above an IAG baseline. If such a signal is detected, the transponder determines if the signal is an IAG trigger and enters IAG protocol communications appropriately.

Such a transponder has limited functionality in that it does not provide for both active and backscatter communications ability. Moreover, in employing parallel detection strategies with separate detection hardware, the transponder is not efficiently scalable to increased protocol implementation.

It is also impractical to install multiple RSE readers implementing different protocols in an attempt to read different transponders implementing different protocols. Each of the reader protocols is designed to utilize the available time efficiently and makes no allowance for sharing the RF spectrum with incompatible RF protocols that could result in RF interference at the transponder or at the reader. Any such attempt can only operate with the penalty of considerable degradation.

Typically, each AVI system uses different identification means for the transponder. In the absence of compatible identification, the opportunity for general mobility of a transponder is further restricted.

It is desirable that a single transponder be capable of responding to a plurality of communications network protocols in a manner that overcomes the limitations of the prior art.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a multi-protocol transponder for communicating with a reader using RF communications in accordance with a plurality of possible protocols. The reader uses a selected one of the plurality of possible protocols. The transponder includes an antenna and an RF modem connected to the antenna for receiving an RF signal induced in the antenna by radiated RF energy from the reader. The RF modem demodulates the received RF signal and outputs a digital signal. The transponder also receives a digital controller for receiving the digital signal and controlling the generation of a response signal in accordance with the selected protocol. The RF modem includes a protocol detector for receiving the RF signal and identifying the selected one of the plurality of possible protocols based upon the radiant energy level of the received RF signal.

In another aspect, the present invention provides a method for operating a multi-protocol transponder communicating with a reader using RF communications in accordance with a plurality of possible protocols. The reader uses a selected one of the plurality of possible protocols. The transponder includes an antenna. The method includes steps of receiving an RF signal induced in the antenna by radiated RF energy from the reader, identifying the selected one of the plurality of possible protocols based upon the radiant RF energy level of the received RF signal, demodulating the received RF signal to obtain a digital signal, and generating a response signal in accordance with the selected protocol.

In one embodiment, transmission may be by either by active transmission or modulated backscatter through a shared antenna.

The transponder may implement a variety of wide area (eg. TDMA ASTM V6 Wide Area) and localized Lane Based protocols (eg. IAG, T21 and TDMA Lane Based ASTM V7) for Automatic Vehicle Identification systems such as CVO and Toll systems. The detector may identify the communications protocol by measuring the energy level of the first RF signal and if the measured level exceeds a threshold trigger level for the protocol, testing the RF signal for communications protocol indicators. A single unique universal transponder ID for use in all protocols may be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.

FIG. 1 is a schematic diagram of a short range radio communication network incorporating a transponder according to the present invention;

FIG. 2 is a simplified block diagram illustrating a protocol selection; and

FIG. 3 is a simplified diagram illustrating a shared antenna for Backscatter and Active Transmission.

DETAILED DESCRIPTION

With reference to FIG. 1, the invention is embodied in a transponder 100 of the type used in short range RF communications with a reader 102 in a reader-transponder communications network 104. Such systems may be useful to automatically identify vehicles on a roadway, reducing restrictions on traffic flow. Readers 102 are typically stationary, set in a suitable location as Roadside Equipment. Transponder 100 is typically mounted in a vehicle that passes within the range of the reader 102 to permit communication and identification of the transponder. It is understood that the term vehicle has a wide application including, a motorized carrier such as a truck, motorcycle, automobile, ship or a transported carrier such as a cargo container. Alternately, the transponder may be stationary and a mobile reader brought within the range of the transponder, for example where an automobile may be parked and a hand held reader drawn near for communications.

Communications between the transponder and reader are governed by a communications protocol that provides the rules of communication. Many protocols exist for such communications and include public TDMA protocols for wide area and single lane communications. In wide area or open road communications, a single reader may communicate with vehicle transponders is several lanes without traffic restrictions. Other protocols are State of California Code of Regulation (CALTRAN) Title 21 (T21) protocol and proprietary IAG protocols.

A simplified diagram of the network 104 that discloses transponder 100 and reader 102 is shown in FIG. 1. Generally, reader 102 comprises an antenna 106, a RF modulator/demodulator 108 and a digital controller 110. Reader 102 is shown connected to an optional host computer 112.

Transponder 100 comprises an antenna 120 and RF modulator/demodulator 122 and a digital controller 124. An optional host computer 126 is also shown connected to the controller 124 as are indicators 128. The optional host computer 126 of the transponder may perform a variety of functions such as computer support and/or memory storage capacity to a protocol controller 130. The host computer 126 may assist to provide an interface to other devices such as Smart Cards, digital recording devices, display devices or the like (all not shown). The indicators 128 may serve to notify the vehicle operator to perform some task, such as, to direct the vehicle to or away from a vehicle inspection station or to note that the balance of a debit account in an attached smart card is below a threshold. Indicators 128 may include LEDs, beepers, etc.

Typically, transponder antenna 120 is located integrally with the transponder 100 and reader antenna 106 is located remote from the reader. In the preferred embodiment, each antenna acts in both a receive mode and transmit mode for RF radiated energy. In the receive mode, each antenna receives and converts radiated energy to a RF voltage signal. In the transmit mode, the reader antenna 106 converts a RF voltage signal to radiated energy. As discussed further below, in the transmit mode, the transponder antenna 120 may convert a RF voltage signal generated under the control of the digital controller 122 to actively transmit radiated energy. Alternatively, according to the protocol sensed by the receiver/protocol detector 134, the transponder antenna 120 may be controlled to modulate a continuous wave transmitted by the reader antenna 106 in a modulated backscatter implementation. Additional control circuitry for Frequency Shift Keying (FSK) sub-carrier modulation may also be used in a backscatter mode.

The RF modulator/demodulator sections 108 and 122 respectively serve to demodulate received RF voltage signals provided by the respective antennas 106 and 120. When transmitting, the RF modulator/demodulator sections 108 and 122 respectively serve to modulate a RF voltage signal prior to being transmitted to the antennas 106 and 120 respectively.

The digital controllers 110 and 124 are each suitably programmed to provide a protocol structure for executing the timing and decision making logic steps necessary to communicate according to the protocol. As discussed more fully below, the transponder controller 124 includes multiple protocol structures to communicate in a variety of protocol networks.

Operation of the transponder 104 will now be discussed with reference to FIGS. 1 and 2. The transponder enters the transmission range of the reader in a quiescent mode Block 200, not engaged in communication with a reader. Radiated energy is received at the antenna 120 and converted to a first RF voltage signal. The first RF voltage signal is presented to a transmit/receive switch 140 located within the RF modulator/demodulator section 122. In the receive mode, the switch 140 routes the first RF voltage signal to a receiver/protocol detector 134. Here, the receiver/detector 134 commences to identify the communications protocol by determining the characteristics of the first RF voltage signal.

In the preferred embodiment, the transponder is configured to implement N communications protocols, three of which are TDMA Wide Area protocol, T21 and IAG referred to as Protocol 1, Protocol 2 and Protocol 3 in FIG. 2. To select which of the N protocols to use in communications with the reader, upon receipt of the first RF signal, the receiver/protocol detector 134 measures the radiant energy level of the first RF voltage signal to determine whether the signal meets a threshold trigger level for any of the plurality of communications protocols understood by the transponder. If the RF level exceeds a threshold indicated as Level 1, Block 205, (greater than about −30 dBm for TDMA Wide Area protocol), the receiver/protocol detector 134 tests the received signal for communications protocol indicators for Protocol 1.

The receiver/detector 134 demodulates the carrier wave (typically amplitude modulated) of the first RF voltage signal to determine if the signal is a demodulated baseband (Manchester) split-phase coded signal. If such is the case, the transponder partially awakens and the protocol detector looks for a Frame Control Message (FCM) signifying TDMA communications from reader 102. If the FCM exists, then the detector selects Protocol 1, Block 260, as the identified communications protocol and notifies the digital controller 124 accordingly to use the protocol, Block 265. Once a protocol is selected, the transponder 100 only responds according to the identified protocol and does not respond to any other protocol triggers. The transponder 100 may revert to a quiescent mode after the expiry of a timer (eg. about 300 ms) in the absence of minimum RF.

If expected communications protocol indicators are not present for Protocol 1, the detector 134 continues to attempt to identify a protocol from the first RF signal. If the RF level of the signal is above a threshold indicated as Level 2 in Block 215 (greater than about −22 dBm for T21), the receiver protocol detector 134 tests the received signal for communications protocol indicators for Protocol 2, Block 220. The receiver/detector 134 examines to see if the RF signal is a wake up pulse of sufficient duration followed within a short period by a properly coded Interrogation or polling message, using Unipolar ASK (Manchester Encoded) modulation, in accordance with T21 specifications. If such is the case, the detector 134 selects Protocol 2, Block 255.

Similarly, if Protocol 2 is not detected, RF Level 3, Block 225, (for example greater than about −10 dBm) is examined and as appropriate, the signal is reviewed for indicators for Protocol 3, Block 230. Protocol 3 is selected in the presence of appropriate indicators in Block 245. Such indicators are the detection of two IAG wake-up triggers in about a 40 msec duration in the absence of a T21 polling message. In a like way, up to N protocols may be determined (Blocks 235, 240 and 250).

Of course, for protocols having the same RF level triggers, they may be handled by looking to differences in communications protocol indicators, such as different Frame Control Messages for different TDMA protocols, to distinguish between protocols. Both the T21 and public TDMA Lane Based (ASTM V6) protocols have the same RF level trigger of greater than about −22 dBm.

It may be necessary that the first RF signal from the reader 102 comprise a set of RF signals (either repeated signals or different signals) for the transponder 100 to determine the protocol. For example, if the transponder 100 receives an incomplete signal, the transponder 100 may be required to receive a repeated reader 102 transmitted signal to obtain sufficient information to determine the protocol. Under many protocols, the first RF signal is repeated frequently to rapidly initiate communications between a reader 102 and a transponder 100. In other protocols, the initiating or first RF signal comprises a set of different signals such as a wake up and an Interrogation signal.

As well as selecting the protocol the receiver/detector 134 provides an analog-to-digital (A/D) conversion of the RF signal as is well known. The demodulated signal is converted to binary data bits and provided to the digital controller 124 together with clock synchronization and an indication of the protocol identified by the receiver/detector 134. The digital controller 124 comprises a protocol controller 130 and a memory 132.

The protocol controller 130 decodes the binary data bits and executes the protocol structure which controls the timing and decision making logic steps of the digital controller 124. Examples of the decision making logic steps or processes performed include timing, message slot selection, memory input/output and transmit/receive control as is well known in the art. For example, T21 is an asynchronous half duplex protocol with downlink (reader to transponder) data ASK (Manchester Encoded) at 600 kBps and uplink (transponder to reader) data FSK encoded at 300 kBps. TDMA and IAG operate downlink and uplink at 500 kBps. A multi protocol transponder must be capable of communicating at the correct data rate.

The digital controller 124 also implements the command features of the message sets encoded in the protocols. Commands are usually incorporated into the protocol as additional data elements. For example, in TDMA, according to ASTM V6 there are controls for the LEDs and beeper which are part of the data fields transmitted to each transponder when properly addressed. Similarly, an IAG compatible transponder may also respond to LCD, LED or beeper commands. In response to certain commands or interrogation signals, data provided by the host computer 126 or the memory 132 is formatted by the protocol controller 130 according to the protocol structure selected in accordance with the present invention.

The memory 132 is connected at the end of a receive path within the digital controller 124. The memory 132 may provide long term storage of standard data for identifying the vehicle transponder 100 to the reader 112. Examples of the data stored in the memory 132 can include the transponder ID, vehicle type, registration number, vehicle identification number, operator identification and license number, vehicle weight, maximum load weight capacity, cargo information such as the bill of lading, ports of entry and the like. Table I illustrates an example of the data fields associated with identification within the three protocols. TABLE 1 Agency Field Size, bits Field IAG Descrip- I- tion IAG Pass TDMA T21 Comment Header 3 X 12 Incompatible headers Public ID X 1 X PIP Protocol Public ID 1 PILE Link Entry Tag Type 3 2 Incompatible Tag Type coding Applica- 3 X tion ID Group ID 7 X Agency ID 7 6 16 Different allocation systems Manufac- X 5 X turer Public 24 23 32 IAG Serial numbers are Serial No combined with Agency ID. TDMA Serial numbers are combined with Manufacturer ID, PIP, PILE, and Tag Type. T21 serial numbers are unique. Private X 32 X Internal knowledge, never ID transmitted Note: X = not used.

Thus, the transponder 100 may store multiple, different identities (IDs), each ID being dependent and associated with a protocol. Thus a multi protocol transponder 100 may be registered with various relevant agencies responsible for CVO and/or ETTM systems. A vehicle with a multi protocol transponder 100 may register its transponder ID as a TDMA transponder for purposes of Weigh Station Bypass in interstate commerce, and register the same transponder with an IAG agency under an IAG transponder ID for purposes of tolling and also register the transponder with a T21 tolling agency all independently.

Protocol formatted digital data generated by the protocol controller 130 is directed to one of two transmit controllers 136,138 associated with the protocol identified. The preferred transponder 100 is capable of both active transmission or modulated backscatter transmission. In the active transmission mode, the formatted digital data is utilized to modulate a transmit oscillator carrier wave to convert the data from a digital-to-analog (D/A) format. The active transmission controller 136 produces a second (typically amplitude modulated) RF voltage signal power amplified for transmission to the transmit/receive switch 140. The transmit/receive switch 140, which is controlled in accordance with the protocol structure, routes the second RF voltage signal to the antenna 120. The antenna 116 then converts the second RF voltage signal to radiated energy which is thereafter transmitted to the reader 112. Alternately, for example if the protocol structure identified is T21, transmission of the second RF signal by the modulated backscatter transmission controller 138 and a backscatter modulator 139 is performed by modulated backscatter of a continuous wave generated by reader 102 as is well known in the art. Reflection modulation acts to cut the reception sensitivity of the antenna 120 by altering the antenna impedance, typically by switching the antenna termination between a matching impedance an applied short circuit (FIG. 3).

While a single transponder antenna 120 is shown, multiple antennae may be used for example to separately receive and transmit or to transmit by active or backscatter methods.

Advantageously, use of a multi protocol transponder 100 described herein with such readers is transparent to the reader.

While the transponder 100 may hold IDs unique to each protocol it may implement, the individual protocol IDs cannot necessarily define a unique ID for the transponder 100 when considered in a global context. A convenient and universal means is required to uniquely identify the transponder, independently of the protocol used. This single unique universal serial number may be incorporated into the transponder 100 for inclusion in a data field for transmission to a reader 102, if requested. The system of individual protocol IDs is convenient for agencies to operate independently from one another; however, this becomes more cumbersome once the agencies begin to exchange account information in a move to achieve interoperability. In such a case, it is convenient to reference the transponder 100 by a single unique identity common to all agencies regardless of protocol. Subsequent record transactions and maintenance could utilize the single unique universal ID.

Preferably, the single universal unique ID may be constructed independently of the other protocol IDs and satisfy the standards requirements such as defined in IEEE P1455. However, the single unique universal ID may be constructed by concatenating elements from each of the various protocol IDs. Within the IEEE P1455 standard, there is described a standard for numbering transponders that incorporates a Manufacturer's ID, Service Agency, and serial number which may be concatenated into a single unique universal ID: Service Agency: 16 bits Serial Number Type: 4 bits Manufacturer Identification: 16 bits Serial Number: 20 bits Total 56 bits

The multi protocol transponder 100 herein described may be used in both Toll and CVO systems. In Toll operations, it is important to localize vehicles by localizing transponders mounted in consistent positions on vehicles and eliminate confusion between adjacent or closely spaced vehicles. For CVO operations, it is important to detect every commercial vehicle as it nears a by-pass station and determine if the vehicle should be allowed to pass or not in time to provide the operator with appropriate instructions. CVO operations are typically conducted under a “Wide Area” mode protocol, where the vehicle location is not critical as long as the vehicle's transponder ID is captured for reference. Toll operations are done within a confined physical area so as to reduce any confusion as to which vehicle transponder is being addressed.

The operating characteristics of transponders and protocols are selected to match the primary application. Transponders designed for CVO use are generally designed to provide Wide Area coverage with relatively limited control over the downlink or uplink parameters of the RF protocol. Transponders designed for Toll, on the other hand, are designed to provide precise control of either the uplink or downlink parameters, such as to provide exact transponder location. A transponder that is capable of reacting to both Wide Area and localized protocols, using appropriate IDs for each operation, is therefore ideally suited for CVO operations that also require Toll operations.

CVO operations are nationally implemented in a compatible manner in the TDMA Wide Area mode of ASTM V6, whereas Toll systems tend to be regionally governed. A multi protocol transponder implementing the Wide Area protocol and several regional protocols (eg. T21, IAG, Lane Based ASTM V6) can provide multi functionality in many geographical areas.

Some Toll applications require time-consuming reader and vehicle communications and processing including confirmation that the processing has applied to specific vehicles as they transit a control point. For example, in a Smart Card Tolling system, it may be necessary to identify the Smart Card, the vehicle class, the Smart Card Cash Balance, perform a mutual authentication between the Smart Card and RSE, calculate the debit amount, start the debit process on the Smart Card, and await the debit confirmation and then correlate the above information with a physical vehicle presence as it passes through a toll location. The physical correlation is required so that enforcement means can distinguish between vehicles that carry Smart Cards and have been successfully debited from those vehicles that do not carry Smart Cards or have not been successfully debited. In existing Wider Area systems, the localizing functions are performed by separate equipment, typically designed to provide angle of arrival calculation by phase comparisons.

A transponder with both Wide Area and Lane Based protocols is ideal for implementing these applications without the need for an external locator subsystem. The Wide Area protocol can be used to initiate the applications processes, using a common ID for reference, and a localized Lane Based transaction can be used to close the transaction. The common ID can be the single unique universal ID as discussed or, it can use the ID associated with the local protocol to simplify system integration.

In the current invention, TDMA Wide Area protocol of ASTM V6 may be used with a localized protocol for such a system (eg. TDMA Lane Based ASTM V7, T21, IAG). Both T21 and IAG protocols are well suited to localization of the transponder. IAG provides localization by providing very strict control of the downlink RF margins, while T21 provides localization by virtue of the rapid attenuation of link margin as a function of range caused by the backscatter characteristics.

Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

1. A multi-protocol transponder for communicating with a reader using RF communications in accordance with a plurality of possible protocols, the reader using a selected one of the plurality of possible protocols, the transponder comprising: an antenna; an RF modem connected to the antenna for receiving an RF signal induced in the antenna by radiated RF energy from the reader, the RF modem demodulating the received RF signal and outputting a digital signal; and a digital controller for receiving the digital signal and controlling the generation of a response signal in accordance with the selected protocol, the RF modem including a protocol detector for receiving the RF signal and identifying the selected one of the plurality of possible protocols based upon the radiant energy level of the received RF signal.
 2. The multi-protocol transponder claimed in claim 1, wherein the plurality of possible protocols includes at least three candidate protocols and wherein said protocol detector includes means for measuring the radiant RF energy against a first threshold and identifying the presence of the RF communications based upon the radiant RF energy exceeding the first threshold.
 3. The multi-protocol transponder claimed in claim 2, wherein said protocol detector includes means for testing the RF signal for indicia of one of said candidate protocols and, based on said testing, either selecting said one of said candidate protocols or comparing the radiant RF energy against a second threshold, and means for testing the RF signal for indicia of a second of said candidate protocols if the radiant RF energy exceeds said second threshold.
 4. The multi-protocol transponder claimed in claim 1, wherein the plurality of possible protocols includes two or more candidate protocols and wherein said protocol detector includes means for excluding protocols as candidate protocols based upon testing the radiant RF energy against a set of escalating thresholds.
 5. The multi-protocol transponder claimed in claim 4, wherein the means for excluding includes means for identifying indicia of a particular protocol within the RF signal based upon finding that the radiant RF energy exceeds one of said escalating thresholds.
 6. The multi-protocol transponder claimed in claim 1, wherein the RF modem includes an active transmitter component for sending the response signal by active transmission and a backscatter modulation component for sending the response signal by backscatter modulation, and wherein the digital controller selects between the active transmitter component and the backscatter modulation component based upon the protocol identified by the RF modem.
 7. The multi-protocol transponder claimed in claim 1, wherein the plurality of possible protocols include a TDMA protocol, a Title 21 protocol, and an IAG protocol.
 8. The multi-protocol transponder claimed in claim 1, wherein the reader operates within an electronic toll collection system, the RF communication relate to a toll transaction, and the multi-protocol transponder is associated with a vehicle.
 9. A method for operating a multi-protocol transponder communicating with a reader using RF communications in accordance with a plurality of possible protocols, the reader using a selected one of the plurality of possible protocols, the transponder including an antenna, the method comprising the steps of: receiving an RF signal induced in the antenna by radiated RF energy from the reader; identifying the selected one of the plurality of possible protocols based upon the radiant RF energy level of the received RF signal; demodulating the received RF signal to obtain a digital signal; and generating a response signal in accordance with the selected protocol.
 10. The method claimed in claim 9, wherein the plurality of possible protocols includes at least three candidate protocols and wherein said step of identifying includes measuring the radiant RF energy against a first threshold and identifying the presence of the RF communications based upon the radiant RF energy exceeding the first threshold.
 11. The method claimed in claim 10, wherein said step of identifying further includes steps of testing the RF signal for indicia of one of said candidate protocols and, based on said testing, either selecting said one of said candidate protocols or comparing the radiant RF energy against a second threshold, and a step of testing the RF signal for indicia of a second of said candidate protocols if the radiant RF energy exceeds said second threshold.
 12. The method claimed in claim 9, wherein the plurality of possible protocols includes two or more candidate protocols and wherein said step of identifying includes a step of excluding protocols as candidate protocols based upon testing the radiant RF energy against a set of escalating thresholds.
 13. The method claimed in claim 12, wherein the step of excluding includes a step of identifying indicia of a particular protocol within the RF signal based upon finding that the radiant RF energy exceeds one of said escalating thresholds.
 14. The method claimed in claim 9, further including a step of transmitting the response signal, wherein the step of transmitting includes modulating the response signal with an RF carrier signal.
 15. The method claimed in claim 14, wherein the transponder includes an active transmitter and a backscatter modulation transmitter and wherein said step of modulating includes selecting between the active transmitter and the backscatter modulation transmitter based upon the protocol identified in said identifying step.
 16. The method claimed in claim 9, wherein the plurality of possible protocols include a TDMA protocol, a Title 21 protocol, and an IAG protocol. 